Studies of heavy helium isotopes are currently in the center of experimental activity . The characteristics of these exotic nuclei allow us to extend our understanding of the properties of nuclear matter under conditions of anomalously high N/Z ratios. A relatively small number of nucleons in heavy helium isotopes makes it possible to describe microscopically their properties and, as consequence, to test nuclear models and nucleon–nucleon potentials.
The ground state of Не (J = 0 ) is the nucleon-stable and is bound to decay into an α-particle and two neutrons at 0.973 MeV . Не is the lightest nucleus that possesses a neutron halo. Also reliable established the existence of narrow first excited state with E = 1.797 (25) MeV, = 113 (20) keV (J = 2 ), which decays into α + 2n . Experimental information on higher excitation states is less certain. In the region of excitation energies E = 2 10 MeV levels were found in the several works ([3,4], the results of earlier works were presented in the compilation ). However, the results of various works poorly agree with each other.
The second group of wide states ( 4 MeV) was found at E 24 MeV [2,3]. The third region is above the threshold of decay He on free nucleons. The indication on two relatively narrow ( 2 MeV) resonances at E = 32 and 36 MeV were obtained in the reaction Li( He, p He) He .
This paper presents an overview of the results on the level structure of He obtained in our studies of the stopped pion absorption by light nuclei. Part of this information obtained in the reactions Be( , tt)t, B( , dd)Х, B( , pt)Х and B( , dt)Х was presented in the works [8,9]. Results of the measurements of the reactions Bе(π , t)X, C(π , d He)X and C(π , d He)X are presented in the first time.
The experiment was performed in the low energy pion beam of meson factory LANL using the two arm semiconductor spectrometer .
A beam of 30 MeV negatively charged pions traversed a beryllium moderator and stopped in the thin target ( 24 mg cm ). The rate of pion stopping in targets was about 6 10 s . In one experimental run the measurements were carried out on the isotope-pure targets Be and C, targets B (contribution of the B impurity was 15%), B (contribution of the C impurity was 8%) and C “radioactive” target (76% is C, 23% is C). The contribution of uncontrolled impurities in all targets was 1%. This set of targets enables the correct estimation of the contribution of the impurities in the measured spectra.
Charged particles, including the hydrogen isotopes p, d, and t and the helium isotopes He, emitted after pion absorption in the targets were detected by two semiconductor telescopes arranged at an angle of 180 with respect to each other. The energy resolution (FWHM) for single-charged particles (p, d, t) was better than 0.5 MeV and 2 MeV for doubly charged particles ( He) . The error of absolute energy calibration did not exceed 100 keV .
A search for the He excited states was carried out on the peaks in the missing mass spectrum (MM). In the measurements of any pairs of singly charged particles the MM resolution was 1 MeV . In the measurements of pairs of single and double charged particles the MM resolution was 3 MeV The error of the MM absolute calibration ( MM) did not exceed 100 keV for pairs of singly charged particles and 200 keV for other pairs .
3. Results and Discussion
In inclusive measurements search for excited states of He was carried out in the reaction Be( ,t)X. The MM spectrum obtained in this reaction is shown in fig. 1.
Peaks formed due to the appearance of the ground state and the excited states of He in the two-body reaction channels are well seen. To resolve these states we used the method of least squares in describing the experimental spectra by the sum of n–particle distributions over phase space (n 3) and Breit–Wigner distributions for excited states. A statistically satisfactory description of the experimental spectrum can be obtained by introducing the ground state and three excited states of He with following parameters (E , ): (1.80(3) MeV, 0.11(2) MeV), (2.5(2) MeV, 0.5(2) MeV) and (4.8(2) MeV, 2.9(2) MeV). Note that the contribution to the measured spectrum the reaction channel + Be t+ Не+n+n is negligible.
The energy of the second excited state coincides with the data obtained in , but the width in our measurements is considerably less. The level with E = 4.8(2) MeV is observed for the first time and lies below the level with E = 5.5(3) MeV, which was observed in this region of excitations previously [3,4]. We have not received indications on the existence of the level with E =3.5(2) MeV, which was observed in reactions of stopped pion absorption by boron isotopes . Perhaps this is due to the difference in the structures of the absorbing nuclei.
We do not observed high excitation states in the measurements of Be( ,t)X reaction. This is caused to the rapid increase of the physical background in inclusive measurements, due to many-particle channels of reaction.
The results obtained in measurements on carbon target are presented in fig. 2. In these measurements MM resolution becomes worse (FWHM 3 MeV). Also in these channels the statistics is not high enough. Therefore in these measurements we observed only the ground state of He and possibly the first excited state in the reaction C( ,t He)X. The inset in the fig. 2 shows the spectrum obtained after subtraction of the sum of the phase-space distributions. It is seen that first excited state allows us to describe the shoulder observed in the peak.
The data obtained in the reactions Be( ,t)X, C( ,d He)X and C( ,t He)X complement information on the region of low excitation energies, which were obtained us in the reaction on boron isotopes: B( , dd)Х, B( , pt)Х and B( , dt)Х . However, due to reasons mainly related to reaction mechanisms, we have not obtained new results on the high-energy excitations.
Summarize the main results for high excitations of He, which were obtained in the reaction Be( , tt)t , and reactions on boron isotopes . High excited states (E 12.3 MeV) were observed in several reaction channels. Some of them were identified as cluster resonances in a system of two tritons. In the reactions B( , pt) He and B( , tt)t there were observed two states with excitation energy lying above the threshold of decay into free nucleons.
The excited states of heavy helium isotopes He were studied in stopped pion absorption by nuclei. Three reactions Be( ,t)X, C( ,d He)X and C( ,t He)X were studied for the first time. New state of He with E = 4.8(2) MeV and Г = 2.9(2) MeV has been observed in the Be( ,t)X reaction. At high excitations levels of He have been found in the reactions Be( , tt)t, B( , dd)Х, B( , pt)Х and B( , dt)Х.
This work was supported by the Russian Ministry of Education and Science grant N3.4911.2017/6.7 and by program of increasing the competitive ability of National Research Nuclear University MEPhI (agreement with the Ministry of Education and Science of the Russian Federation of August 27, 2013, project no. 02.a03.21.0005).